Senate Democrats' campaign arm is hitting the airwaves in the Iowa Senate race beginning Tuesday with a sizeable advertising buy that comes as polls show a close contest between state Sen. Joni Ernst (R) and Rep. Bruce Braley (D).

The committee has reserved more than $500,000 in broadcast air time over the next two weeks in four media markets, according to public records. The buy comes as a pair of recent polls from NBC News/Marist and Quinnipiac University have shown a close race.

"Joni will protect social security for seniors like her parents, and ensure it's there for generations to come, like her daughters and grandchildren," responded Ernst spokeswoman Gretchen Hamel in a statement.

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